1. Field of the Invention
The present invention relates to a method of forming an oxygen- or nitrogen-terminated silicon nanocrystalline structure (porous silicon) and in particular to a novel method of forming a silicon nanocrystalline structure (porous silicon) by using a plasma treatment equipment and to a silicon nanocrystalline structure (porous silicon) formed by the method.
2. Description of the Related Art
Semiconductors of compounds such as gallium and arsenic would be applicable as silicon-based light-emitting elements used in the fields of optical interconnection for optical communication, optical communication and visible-light sources.
However, semiconductors of compounds such as gallium and arsenic hardly produce a structure with fewer defects on a silicon substrate, and are poor in thermostability. Because the existing process of producing silicon integrated circuits cannot deal with such production, a new production process is necessary, thus increasing production costs.
Accordingly, there is demand for techniques of producing silicon-based light-emitting structures that can be produced by only the existing process of producing silicon integrated circuits.
As a silicon-based light-emitting material, a silicon nanocrystalline structure (porous silicon) produced by anodizing is known.
A flow of anodizing treatment, i.e. a conventional process for producing a silicon nanocrystalline structure (porous silicon), is shown in FIG. 5.
In FIG. 5, a polysilicon film is formed to a thickness of about 1 μm on a substrate by CVD (Chemical Vapor Deposition) method, and then the polysilicon film formed by CVD method is subjected to anodizing treatment.
For example, when an electrolytic cell filled with an electrolyte such as an aqueous solution of HF, wherein a semiconductor such as silicon substrate is used as an anode and platinum is used as a cathode, is electrified, electrons are transferred from the anode through an external circuit to the cathode, and oxidation reaction in a general meaning proceeds on the surface of the anode (i.e. the silicone substrate) contacting with the electrolyte.
However, a silicon nanocrystalline structure (porous silicon) produced by the above-mentioned conventional method has problems such as low luminous efficiency and a low luminous extinction rate in the order of microsecond (μsec). Further, the production process is complicated because of its wet process involving an electrochemical treatment, and is hardly applied to a process of producing silicon integrated circuits. There is also a problem that the surface of silicon nanocrystalline structure (porous silicon) thus produced is covered with hydrogen atoms, and it is unstable in composition, and fragile, so that it is easily broken.
Japanese Patent Application Laid-Open (JP-A) No. 2000-273450 proposes a silicon nanocrystalline structure (porous silicon) that can be produced directly on a substrate such as a silicon substrate by the process of producing silicon integrated circuits. In this prior art, a silicon-based light-emitting material having a silicon-rich amorphous structure, based on silicon and nitrogen, is produced from a raw material gas such as silane (SiH4) and an ammonia gas (NH3) in a specific ratio of raw material gas/(raw material gas+ammonia gas) by CVD method at specific temperature.
The conventional method of forming a silicon nanocrystalline structure (porous silicon), shown in the treatment flow in FIG. 5, involves formation, by chemical synthesis, of a nanocrystalline structure of a polysilicon film of 1 μm in thickness produced by CVD and simultaneous oxygen termination of the surface of the said polysilicon film. That is, the formation of a nanocrystalline structure by a polysilicon oxidation/etching process and the oxygen termination of the surface by an oxidization process proceed simultaneously in the chemical treatment. Due to this production process, the particle diameter of the oxygen-terminated silicon nanocrystalline structure is hardly regulated in an accuracy of 1 to 2 nm, thus causing problems such as varying luminescent colors.
The before described JP-A No. 2000-273450 does not refer to the regulation of the particle diameter of the silicon nanocrystalline structure (porous silicon) affecting luminescent colors.
Further, because nanocrystals formed during the chemical treatment may disappear in the subsequent etching/oxidization process, there is the problem of a reduction in density of nanocrystals per unit area. This problem is a major cause of a reduction in luminous efficiency in a porous silicon luminescent system using the silicon nanocrystalline structure (porous silicon) formed by the conventional method shown in the treatment flow in FIG. 5.